Abrasive tools are generally classified according to the form in which the constituent ceramic grains thereof are formulated: free abrasives (use in spraying or in suspension, without a support), coated abrasives (support of cloth or paper type, where the grains are positioned over several layers) and bonded abrasives (in the form of circular grinding wheels, sticks, and the like). In the latter cases, the abrasive grains are compressed with an organic or glass binder (in this case, the binder composed of oxides which is essentially silicated). These grains must themselves exhibit good mechanical properties in the abrasion (in particular toughness) and give rise to good mechanical cohesion with the binder (durability of the interface). Currently various families of abrasive grains exist which make it possible to cover a broad range of applications and of performance: grains of oxides synthesized by fusion in particular offer an excellent quality/manufacturing cost compromise.
Within the range of fused grains, materials based on alumina and zirconia are known from U.S. Pat. No. 3,181,939. These grains are generally composed of from 10 to 60% of zirconia and from 0 to 10% of an additive, the remainder being alumina. In practice, the zirconia content of commercial products is either about 25% or about the value of the alumina/zirconia eutectic located at approximately 42% of zirconia, generally from 35 to 50%, as described in the U.S. Pat. No. 3,891,408. This patent indicates that the products having a composition close to the eutectic offer a better performance in the application than aluminous grains, in particular if they are solidified very rapidly, so that, in the eutectic colonies, the interlamellar or interfiber spaces are less than 4000 Å (with eutectic colonies orientated perpendicular to the solidification front). This unique type of structure produces an excellent compromise between the mechanical strength required for maximum duration of use of the grain and the microfracturing necessary for good regeneration of the cutting surfaces.
Known additives comprise yttrium oxide, added up to 2% according to U.S. Pat. No. 4,457,767, or titanium oxide, added up to 10% according to the patent DE-C1-4306966. These additives improve the abrasiveness of the alumina/zirconia grains. Magnesium oxide is also a possible additive but, above a content of a few points, its presence results in the formation of spinel with the alumina until the corundum disappears, which results in a poor mechanical performance.
Mention may be made, as example of the grains, of the grains described in U.S. Pat. No. 4,457,767. These grains typically comprise 39% by weight of zirconia, 0.8% of Y2O3 and less than 0.5% of impurities, the remainder being alumina. Mixtures of these grains are widely used for coated abrasives or abrasive grinding wheels having an organic binder in operations having a high rate of removal of material (rough grinding, parting, and the like), in particular on stainless steel.
Also known, from FR 2 787 106 or US 2004/0040218, are grains comprising a zirconia content of between 10 and 60% and an Al2O3 content of between 38 and 90%. These fused alumina/zirconia grains are manufactured by molten raw materials in a reducing environment (within particular addition of a source of carbon, for example petroleum coke, pitch or coal, to the furnace). The molten material is subsequently cooled, preferably rapidly, in order to promote the production of fine and oriented structures, for example by means of a device for pouring between thin metal plates, such as that presented in the U.S. Pat. No. 3,993,119. The cooled material is finally ground, for example using roll mills, and then sieved and classified into a series of particle size distributions (“grits”) corresponding to precise standards (for example the FEPA standard). The grains thus manufactured are compact and not very reduced. They exhibit good mechanical properties and can advantageously be used in grinding wheels having a glass binder.
Mixtures of fused alumina/zirconia abrasive grains are also described in FR 2 872 157. They exhibit noteworthy abrasive properties, in particular under certain low-pressure grinding conditions on a stainless steel or more generally on a steel having a high carbon content and of high hardness.
The abrasive properties of the abrasive tools decline as they are used, until they are rendered unemployable.
U.S. Pat. No. 5,161,696 discloses the use of blocky brown fused alumina grains exhibiting an aspect ratio of less than 2:1 for bonded abrasives, the powders used comprising, however, between 20% and 30% of sharp brown fused alumina grains exhibiting an aspect ratio of greater than 3:1. For coated abrasives, this document discloses the use of sharp brown fused alumina grains, the powders used being able, however, to comprise up to from 30% to 40% of blocky grains.
There thus exists an ongoing need for a mixture of fused alumina/zirconia abrasive grains exhibiting a greater lifetime. One aim of the embodiments of the present disclosure is to meet this need.